High reactance transformer



Nov. 19, 1963 N. OULETTA HIGH REACTANCE TRANSFORMER 2 Sheets-Sheet 1 Filed April 11. 1960 Nov. 19, 1963 N. OULETTA HIGH REACTANCE TRANSFORMER Filed April 11, 1960 2 Sheets-Sheet 2 United States Patent ()fi ice 3,111,637 Patented Nov. 19, 1963 3,111,637 HKGH REACTANCE TRANdFQRMER Nichoias Quletta, Meirose Park, iii, assignor to Jefferson liiectric Company, llteliwood, REL, a corporation of Deiawnre Filed Apr. Ill, 1960, Ser. No. 21,526 8 Claims. (sCl. 336-165) This invention relates to high reactance transformers such as those used as ballasts for gaseous discharge lamps in a leading circuit, and in particular to an improved core structure.

It is well known that in such devices, the secondary core portion tends to saturate, providing undesirable wave shapes, unless it is gapped so as to prevent such satura tion.

'It has heretofore been the practice to provide a bridged gap in the secondary core portion in the form of a slot, although this is not an entirely satisfactory substitute for a full gap because the bridges of the slot in themselves become saturated and produce a peaked voltage wave shape. In an effort to minimize the effect of the peak, the slots have been made long and narrow.

The disadvantage of a iong and narrow slot is that it reduces the coupling factor and also weakens the core portion in which it is punched especially when the length of the slot is 75% to 80% or more of the width of the core portion. Also, a long narrow punch of generally rectangular shape is difficult to make and costly to maintain.

It is an object of my invention to provide an improved core structure which overcomes the above disadvantages.

According to my invention, I have found that the desired magnetic reluctance can be obtained by the use of a bridged gap which is in the form of a circular gap punched into the core portion, and that the circular gap provides a more favorable wave shape as well as certain other advantages.

For instance an equivalent R.M.S. voltage can be obtained with a circular gap the diameter of which is considerably less than that dimension of the slot which is transverse to the core portion, thus providing a core portion of greater mechanical strength.

I have also found that the manufacturing cost of a circular gap is reduced because the circular punch is easier to make and cheaper to maintain.

I have also found that design technique is considerably simplified by the use of a circular slot.

Another object of my invention is to provide an improved bridged gap for a high reactance transformer.

Still another object is to provide in a high reactance transformer adapted to supply arc discharge devices in a leading circuit, an improved double gapped core structure which permits the use of a lower turn ratio without sacrifice of other desirable characteristics.

The present application is a continuation in part of my copending application, Serial No. 647,771, filed March 22, 1957.

Other objects, features and advantages will become apparent as the description proceeds.

With reference now to the drawings in which like reference numerals designate like parts:

FIG. 1 is a plan view of a preferred embodiment of my invention showing the core structure of a trans-former, the location of the primary and secondary windings being shown in dotted lines;

FIG. 2 is a plan view of a modification;

FIG. 3 is a diagram showing the voltage Wave shape obtained with a rectangular slot;

FIG. 4 is an enlarged diagrammatic view of a bridged gap formed in accordance with my invention and illustrating the theory of operation;

FIG. 5 is a diagram similar to FIG. 3 but showing the voltage wave shapes obtained by the practice of my invention; and

FIG. 6 is an electrical diagram of a circuit embodying my invention.

With reference now to FIG, 1 which shows the core structure of the high reactance transformer, reference numeral 10' designates the winding leg, and the reference numerals 11 and 12 designate the yoke portions. These elements are built up of laminations of the T and L types.

The resulting shell type core structure provides windows 13 in which are disposed the primary winding 14 and the secondary winding 15, both shown in dotted lines. In the particular design shown, no separate shunt portion is provided, since a leakage path 19 is provided by the mere juxtaposition of the two windings, although a magnetic shunt type of path may be provided if desired. Actually, the windings are spaced from each other by about a three-eighths of an inch, since in win-ding the coils the paper separator extends beyond each coil layer by approximately one-eighth inch to provide a margin, and the coil margins may be separated by a space of about one-eighth to one-quarter inch.

Those portions of the winding leg It which are surrounded by the primary and secondary windings can be referred to as the primary core portion 16, and the secondary core portion 17, respectively.

A bridged gap is located in the secondary portion 17 at about the halfway point, the bridged gap comprising a circular opening 13 which is punched in the winding leg 10' having a diameter of approximately 65% of the width of the winding leg, and which provides saturable bridges 2-1.

The primary winding 14 is connected to a suitable source of power, and the secondary winding 15- is connected to a leading load, either directly, as in the case of an insulated transformer, or in autotransformer relationship. The circular gap 13 prevents saturation of the secondary core portion 17, controls the load current, and provides a smooth wave shape of both voltage and current, as explained below.

In the modification of FIG. 2, an auxiliary circular gap 20 is provided which improves the distribution of leakage flux, depending upon its location, and which cooperates with the main gap 18 :to provide the desired amount of reluctance.

The use of the circular gap 18 considerably reduces the cost of manufacture because a circular punch is cheaper to make than a rectangular punch and it is easier to maintain. Also, the use of the circular gap 18 considerably simplifies technique of transformer design as contrasted with the use of a rectangular slot, since only one variable, the diameter of the circular gap, is involved whereas in the case of a slot there are two variables, length and width.

For example, the effect of the slot dimensions on the open circuit voltage wave shape of the secondary winding 15 is shown in FIG. 3 in which the curve 22 represents the flux in a portion of the core adjacent to the slot, and curve 23 indicates the open circuit induced voltage in the secondary. The dottedline 24 represents the sine wave voltage which would be induced if the slot were not present, the area between lines 24 and 23 representing peak 25 is caused by the low reluctance of the iron bridges up to the point of saturation; after saturation,

3 the reluctance of the bridges approaches that of air with the result that during the saturated portion of the half cycle, the bridged gap functions as a full gap of a definite width, equal to the slot width.

It will therefore be seen that in designing to meet a particular R.M.S. voltage, not only must the slot width be considered as in full gap design, but also the length of the slot because the area of the peak contributes materially to the R.M.S. voltage.

Furthermore the steepness of the peak causes undesirable deformation of the current wave shape. The steepness of the peak is caused by abrupt changes in the reluctance of the magnetic path;

-I have found that by the use of a diiferently shaped slot, these abrupt changes in reluctance can be ameliorated in order to provide a smoother voltage wave, and at the same time, design technique can be considerably simplified.

Although :the exact theory is not fully understood, reference to FIGS. 4 and 5 illustrate what I presently believe to be the proper explanation.

In FIG. 4 the circular gap is is shown in the winding leg Here the bridge portion 21 is of varying transverse dimension, and can be considered as being made up of a number of saturable portions of which several are indicated by different types of hatching, the shortest being portion 30', the next shortest being portion 31 and the longest being portion 32. The successive saturation of portions 30 and 31 and 32 could be considered as producing a segmental flux curve 33 as shown in FIG. 5.

The open circuit voltage wave shape associated with the segmental flux curve 33 is diagrammatically represented by the stepped curve 34 of FIG. 5.

Thus the ordinary bridged gap in effect provides, after saturation, the reluctance of a full gap of a width equal to the width of the slot. However, a circular gap, after initial saturation, acts magnetically as a full gap of variable width, the width and the reluctance increasing according to the excitation.

A comparison of curves 23 of FIG. 3 and 34 of FIG. 5 shows that this variable reluctance considerably modifies the wave shape. As indicated by the dotted line curve 35 of FIG. 5, which would correspond to the selection of an infinite number of infinitesimally narrow saturable portions, it will be seen that the voltage wave shape is considerably smoothed out. This results in a much smoother current wave shape than that which would be produced by the peaked voltage curve 23 of FIG. 3. Furthermore, the very substantial taper of the voltage curve 35 tends to produce a square current wave shape.

A comparison of the areas under the curves 23 and 84 shows that in order to produce a given R.M.S. open circuit voltage in a given core structure, that the diameter of the circular gap will be considerably greater than the width of the corresponding slot, and that the diameter will be somewhat less than the length of the slot. This results in a stronger core part, and also in a closer magnetic coupling between the primary and secondary windings which in turn reduces the number of turns required for the secondary.

Furthermore, I have found that by varying the diameter of the circular gap with respect to the width of the core portion 10', I am enabled to obtain any desired open circuit R.M.S. voltage in the same manner as is possible by varying the width of a full gap. In other words, only one variable is involved in design problems for a given set of parameters. In designing with a slot, however, there are two variables, the length, and the width, as above indicated, with the result that design technique is much simplified by the use of a circular gap.

Furthermore, in all instances, the shape of the voltage wave in the case of a circular gap is very much smoother than in the case of a slot.

In general, the desired combination of reluctance and 4 favorable wave shape is obtained only when the diameter of the circular gap is between substantially 50% and to of the width of the core part in which it is formed. However, when an auxiliary gap is used, as in FIG. 2, the diameter of the auxiliary circular gap may be somewhat less than 50%.

The ballast circuit of FIG. 6 illustrates an application of my invention, in which the primary and secondary windings 14 and 15 are connected in autotransfor-rner relationship and in which an operating condenser 44 and two rapid start lamps 41 and 42 are connected in series circuit therewith. Filament windings 43, 44 and 45 may be overwound on the primary 14 in the customary manner, their connections to the lamp filaments being standard and not shown herein. Also a starting condenser of .95 mfd. may be connected across lamp 41 in the customary manner. The elements 43 to 46 do not affect the operation of my invention as outlined above.

The primary 14 comprises 760 turns of #27 wire, and the secondary 1342 turns of #28 wire. They are mounted on a double gapped winding leg ll? of a laminated core having the same general proportions as that shown in FIG. 2.

The overall core dimensions are 4.7 inches by 2.1 inches, and the stack is inch high.

The Winding leg is .812 inch wide. The main circular gap 18' is .437 inch in diameter and the center line is offset from the flux leakage path 19 by substantially 66% of the length of the secondary core portion 17. The auxiliary slot 20 is .375 inch in diameter and the center line is offset from the flux leakage path by substantially 12% of the length of the secondary core portion 17. Thus the diameters of the circular gaps are approximately 54% and 47%, respectively, of the width of the winding leg.

The operating condenser is 4 mfd. and the lamps are 430 ma. rapid start 40 -watt lamps, type 48T12. When the primary is connected to a 118 volt 6O cycle line, the current in the secondary circuit is .381 ampere having a crest factor (peak to R.M.S. ratio) of 1.5. The line current is .787 ampere and the primary current is .516 ampere. Power factor is 97.6% and light output is 99% (ASA test).

The voltage drops across points A, B, C, and D are as follows:

AB 118 AC 301 AD 207 BC 197 BD 149 CD 247 The open circuit voltage across points AC is 295 volts and has a crest factor of 1.59. The exciting current is .363 ampere on open circuit.

The subject matter of the aforesaid copending application is incorporated herein by reference, insofar as consistent with the present disclosure.

Although only preferred embodiments of my invention are shown and described herein, it will be understood that modifications and changes can be made in the embodiments shown without departing from the spirit of my invention as pointed out in the appended claims.

I claim:

1. A high leakage reactance transformer comprising a primary winding, a secondary Winding, and a core structure, the core structure having a winding leg and a yoke portion, and the primary and secondary windings being mounted axially adjacent to each other on said winding leg to provide a flux leakage path therebetween, said winding leg being divided into primary and secondary portions by said flux leakage path, and having a bridged gap formed in said secondary portion, said bridged gap comprising a circular opening having a diameter of between 50% and 80% of the width of said winding leg, whereby the bridge portions adjacent said opening are of varying cross sections whereby the overall reluctance of the gapped portion of said winding leg varies with the magnetizing force.

2. A capacitive type series ballast for a fluorescent lamp comprising a core structure which includes a winding leg, at bridged gap formed in said Winding leg and comprising a circular opening having a diameter of more than half of the width of said winding leg, a secondary winding mounted on said winding leg and completely surrounding said bridged gap, and a primary winding mounted on said winding leg but displaced axially from said secondary winding.

3. A ballast as claimed in claim 2 in which said primary and secondary windings are connected in autot-ransformer relationship.

4. A ballast as claimed in claim 2 in which said core structure has an auxiliary bridged gap comprising a circular opening having a diameter less than that of said first mentioned circular opening, said auxiliary bridged gap being located between said first mentioned bridged gap and said primary winding.

5. A capacitive type series ballast for a fluorescent lamp comprising a primary winding, a secondary Winding, and a core structure, the core structure having a winding leg and a yoke portion, and the primary and secondary Windings being mounted axially adjacent to each other on said winding leg to provide a flux leakage path therebetween, said winding leg being divided into primary and secondary portions by said flux leakage path, and having a bridged gap formed in said secondary portion, said bridged gap comprising a circular opening having a diameter of between 50% and 70% of the width of said winding leg and being located at a point offset from said flux leakage path by at least 50% of the length of said secondary portion whereby the overall reluctance of the gapped portion of said winding leg varies with the magnetizing force to provide improved wave shape of induced voltage and load current.

6. A ballast as claimed in claim 5 in which said core structure has an auxiliary circular gap formed in said secondary portion at a point offset from said leak-age path by substantially 12% of the length of said secondary portion.

7. A ballast as claimed in claim 6 in which the diameter of said auxiliary circular gap is between and of the width of said winding leg.

8. In a leakage reactance transformer having a core structure which includes a winding leg, a bridged gap formed in said winding leg, a secondary winding mounted on said winding leg and surrounding said bridged gap, and a primary winding mounted on said winding leg but displaced axially from said secondary winding, the combination of a pair of parallel bridges for said bridged gap, each having a length of more than one half the width of said winding leg, and each bridge including a portion of progressively increasing cross section whereby the overall reluctance of that portion of said winding leg which is surrounded by said secondary winding will vary with the magnetizing force.

References Cited in the file of this patent UNITED STATES PATENTS 1,920,818 Verrall Aug. 1, 1933 2,432,343 Short Dec. 9, 1947 2,476,419 Koenig July 19, 1949 2,664,541 Henderson Dec. 29, 1953 2,799,822 Dewitz July 16, 1957 2,869,037 Brooks et al. Ian. 13, 1959 2,947,909 Berger Aug. 2, 1960 FOREIGN PATENTS 616,428 Great Britain Jan. 21, 1949 

2. A CAPACITIVE TYPE SERIES BALLAST FOR A FLUORESCENT LAMP COMPRISING A CORE STRUCTURE WHICH INCLUDES A WINDING LEG, A BRIDGED GAP FORMED IN SAID WINDING LEG AND COMPRISING A CIRCULAR OPENING HAVING A DIAMETER OF MORE THAN HALF OF THE WIDTH OF SAID WINDING LEG, A SECONDARY WINDING MOUNTED ON SAID WINDING LEG AND COMPLETELY SURROUNDING SAID BRIDGED GAP, AND A PRIMARY WINDING MOUNTED ON SAID WINDING LEG BUT DISPLACED AXIALLY FROM SAID SECONDARY WINDING. 